The present invention relates to a method and apparatus for burning a fuel in the presence of oxygen and air. More particularly, the present invention relates to such a burner in which the combustion is staged such that the fuel initially bums at a fuel-rich stoichiometry and then at a fuel-lean stoichiometry in first and second stages of combustion, respectively. Even more particularly, the present invention relates to such an apparatus and method in which oxygen is used to support combustion in the first stage of combustion and swirled air or oxygen enriched air is employed to support combustion in the second stage of combustion.
In many industrial processes it is necessary to heat a melt, for instance, glass, metals, ferrous and non-ferrous and etc. A variety of burners have been provided for such processes that bum a fuel in the presence of an oxidant that can consist of air, oxygen, or oxygen-enriched air.
Burners that use air alone can generally be said to suffer from a limited operating range, power output and a potential for NO.sub.x formation resulting from combustion of the fuel in airborne nitrogen. In order to reduce NO.sub.x formation, U.S. Pat. No. 4,297,093 provides a burner in which air is swirled around a fuel jet to produce a fuel rich zone of combustion surrounded by a fuel lean zone of combustion. Combustion of the fuel in such a manner reduces NO.sub.x formation and the swirling air acts to insolate the burner from the flame.
In order to increase the operating range and available power output of the burner, oxy-fuel burners have been developed that utilize oxygen as the oxidant. Such burners are compact and are able to produce a high energy output from both the rate at which fuel is burned and the luminosity of the flame. Their disadvantage is that the flame they produce can result in high localized heat fluxes that develop hot spots in the melt. Moreover, the use of oxygen adds to the costs of the process involved. In order to conserve oxygen, burners termed in the art as air-oxy-fuel burners have been developed in which a portion of the oxidant is supplied by air to reduce the costs involved in burning a fuel in oxygen alone. A potential problem with such burners, is that they are more likely to produce high amounts of NO.sub.x than burners that use just air to supply the oxidant.
An example of an air-oxy-fuel burner is U.S. Pat. No. 5,145,361 in which air is aspirated in either a flow of fuel gas or oxygen and the resultant mixture is swirled to spread out the area in which the flame bums. A high potential for NO.sub.x formation exists in the type of combustion contemplated in the '361 patent. U.S. Pat. No. 4,642,047 discloses another air-oxy-fuel burner in which fuel and oxygen are burned at a fuel-rich stoichiometry in the core of the flame and air is tangentially introduced about the flame core to blanket the flame for insulating purposes and to complete combustion of the fuel. The lack of air in the core of the flame inhibits NO.sub.x formation. The '047 patent is not unlike the '093 patent discussed above. Both of these patents contemplate radially stratified fuel rich and fuel lean zones to lessen NO.sub.x formation. In producing such stratified zones, however, the ability of the flame to be outwardly projected is greatly diminished over burners in which fuel and oxidant mix in an axial direction of the flame. As a result, also the '047 patent requires water cooling to prevent destruction of the burner resulting from intense burning in close proximity to the burner. Forward projection of the flame away from the burner is essential to provide for good heat transfer to a thermal load and allows self cooling of the burner.
As will be discussed, the present invention provides an air-oxy-fuel burner that is able to project a flame at a lower oxygen consumption than the burners set forth in patents mentioned above and which is designed to inhibit NO.sub.x formation.